[html] How to edit HTML input value colour?

You can change the CSS color property using JavaScript in the onclick event handler (in the same way you change the value property):

<input type="text" onclick="this.value=''; this.style.color='#000'" />

Note that it's not the best practice to use inline JavaScript. You'd be better off giving your input an ID, and moving your JavaScript out to a <script> block instead:

document.getElementById("yourInput").onclick = function() {
    this.value = '';
    this.style.color = '#000';
}